Despite taking prompt action to defend against the Heartbleed attack, some sites are no better off than before -- and in some cases, they are much worse off.

Many of the sites that patched vulnerable OpenSSL installations after the Heartbleed attack was revealed on April 7 then went on to revoke compromised SSL certificates and order new ones. But 30,000 sites are now using replacements based on the same compromised private key as the old certificate, according to a study by Internet services company Netcraft released Friday.

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That means that anyone who managed to steal the private key of such a server before it was patched could still use the key to impersonate the server in a man-in-the-middle attack, even with the new certificate in place.

The error is a dangerous one, because the operators of vulnerable servers are likely to believe they have taken all the steps necessary to protect their users, Netcraft warned.

Around 57 percent of sites vulnerable to the Heartbleed attack have so far neither revoked nor reissued their SSL certificates, Netcraft said. A further 21 percent have reissued certificates but not revoked the compromised ones.

The 30,000 sites that revoked their certificates and reissued new ones with the same private key represent around 5 percent of vulnerable sites, according to Netcraft. An additional 2 percent are reusing the same private key and have yet to revoke their old certificates.

Those sites at least are no worse off than the day the Heartbleed attack was revealed.

That's not the case, though, for the roughly 20 percent of servers vulnerable today that were not vulnerable when the attack was revealed: Their operators appear to have replaced safe versions of OpenSSL with flawed ones, according to a study by software developer Yngve Nysæter Pettersen.

"One possibility is that all the media attention led concerned system administrators into believing their system was unsecure. This, perhaps combined with administrative pressure and a need to 'do something', led them to upgrade an unaffected server to a newer, but still buggy version of the system, perhaps because the system variant had not yet been officially patched," he suggested.

Nysæter Pettersen began scanning on April 11, and in the following two weeks around half the vulnerable servers he found were patched, bringing the proportion running vulnerable OpenSSL installations down from 5.36 percent to 2.77 percent. Since then, though, "Patching of vulnerable servers has almost completely stopped," with 2.33 percent still unpatched on Wednesday, he said.